Electron tube and filamentary cathode



July l, 1958 A M. B. SHRADER 2,841,736

ELECTRON TUBE AND FILAMENTARY CATHODE INVENTOR.

MERRELL: B. SHRHDER ORNEY July l, 1958 l M. B. sHRADER 2,841,736

ELECTRON TUBE AND FILAMENTARY CATHODE Filed June 5, 1953 2 Sheets-Sheet2 lijf.

INVEN TOR.

MERRHLD B. SHRHDER United States Patent O ELECTRON TUBE AND FILAMENTARYCATHODE Merrald B. Shrader, Mount Joy, Pa., assignor to RadioCorporation of America, a corporation of Delaware Application June 5,1953, Serial No. 359,770

13 Claims. (Cl. 313273) This invention relates to electron tubes andmore particularly to such tubes having a filamentary type cathode.

Filamentary cathodes are characterized by dependable electron emissionover an exceedingly long useful life. Furthermore, such filaments havesuccessfully been utilized in tubes having a wide range of power outputcapabilities even though relatively complex arrangements are required.While iilamentary cathodes have come tobe more widely used in powertubes than indirectly heated or unipotential cathodes, this is not thecase with respect to high frequency tubes. The flamentary cathodestructures heretofore utilized, in addition to having certain mechanicaldisadvantages, have also been unsuitable for use at the higherfrequencies.

One of the disadvantages of the structures heretofore utilized may bebest pointed out in connection with the so-called self-supportinglamentary structure. In such a structure lamentary strands are usuallyarranged around a circle with adjacent strands extending parallelV toone another. The adjacent strands are connected at one end to differentheating current lead-ins while at their opposite ends the strands arebound together to form the self-supporting structure. The heatingcurrent which heats the filament strands to electron emissiontemperature travels up alternate strands to the bound ends and then downthe remaining strands. One of the filament heater lead-ins serves alsoas the radio frequency current lead-in thereby resulting in the radiofrequency current traveling up alternate strands and then down theremaining strands. Obviously such an arrangement is limited torelatively low frequencies since otherwise the electrical phasedifference' between adjacent strands would be found objectionable.Furthermore, the low frequency heating" current e. g., 60 cycles,usually utilized adversely affectsA the operation of such tubes and itsharmful effects must be overcome. Often this is extremely diflicult andcostly to carry out.

Some of the foregoing short comings are ameliorated by utilizing acentral support connected to the bound ends or electrical center of thefilamentary strands. The center tap thus provided may be utilized astheradio frequency' lead-in or connection to the filament structure.However, such a structure also is limited to relatively low frequency.As is well known, with increasing frequency the elements of a tubebecome more and more a significant part of the high frequency circuit.Theinduct'ance of leads and filament strands becomes a major portion ofthe circuit and effectively limits the upper frequency of operation.

While the filamentary type of cathode structure is widely used, thestructures utilized have often proven to be: fragile and subject todeformations which have limited their usefulness.

It is therefore a principal objectof my invention to providean improvedelectron tube having a` substantially unipotential filamentary cathode.

Another important object is to provide an electron tube having asubstantially unipotential lamentary cathode and capable of operation athigh frequencies.

Still another object is to provide an electron tube having asubstantially unipotential lamentary cathode capable of operation athigh frequencies and high power. Another object is the provision of anelectron tube in which the filament heating current leads arecornpletely shielded from the radio frequency current leads and do notthemselves carry any radio frequency current. Yet another object is theprovision of an electron tube having a substantially unipotentiallamentary cathode and having reduced overall dimensions.

Stated generally, I provide a pair of filament heating current lead-inswhich extend into the electron tube f envelope and to each of which areconnected filament support rods. Alternate support rods are connected toone lead-in while the remaining support rods are connected to the otherlead-in. I utilize filament strands which are doubled over or joinedtogether at one end so as to form interconnected pairs. The` free endsof the doubled over or paired filament strands are each connected toadjacent support rods while the strands themselves extend back along thesupportrods. A tubular radio frequency 1eadin is sealed through theenvelope so as to completely shield the heating current lead-'ins fromthe remaining lead-ins and elements of the tube which may includecontrol electrode, screen electrode, and anode lead-ins and electrodes.The filamentary` cathode radio frequency lead-in extends into theenvelope and` is conductively connected to each of the filament strandsat their electrical center. In such an arrangement the radio frequencycurrents travel up all of the filamentary strands in phase and do nottravel along the filament support rods at all. Thus the electricallength of the cathode structure with respect to radio frequency is thato'f the radio frequency lead-in and the filament strands themselves. Theelectrical length of the support rods is not included at all.

My invention as well as further objects and advantages thereof will bemost clearly understood from the following description thereof and theaccompanying drawings in which:

Figure 1 is a sectional view of an electron tube constructed inaccordance with my invention;

Figure 2 is a fragmentary sectional view through the line 2 2 of Figurel;

Figure 3 is a fragmentary perspective view of a portion of the filamentstructure;

Figure 4 is a sectional view of yet another filament and filamentsupport structure constructed in accordance with my invention; and

Figure 5 is a fragmentary perspective view of a filament structuresimilar to that shown in Figure 4.

Referring now to the drawings in detail and to Figures 1, 2 and 3 inparticular, electron tube 10 comprises a filamentary cathode 11, anode12, and may also include control electrode 13 and screen electrode 14.The control and screen electrodes 13, 14 themselves` may be of anysuitable form as is well known in the art and need not be described indetail here. Anode 12 in the present instance forms part of the`envelope of tube 10 and may be provided with an array of cooling fins15, as shown, to form a radiator through which a coolant such as air maybe forced under pressure.

Extending into the recess formed by anode 12 are a plurality of filamentsupport rods 16, and 16a which may be molybdenum and are arrangedalternately in an annular array ini the present instance. All of thesupport rods 16 are joined at one end as by welding to a support ringl17 which may be of molybdenum or tung,- sten or the like. Support rods16a extend'through `holes or recesses in support ring 17 and are joinedto a similar support ring V18. A pair of nested coaxial tubularleadins'and supports which may be copper are sealed through the envelopeof tube 10. Lead-in and support 19 is joinedat -its-inner end Vtosupport ring 17 whilelead-in andfsupport `20 isjoined to support ring18. Insulating ,support means may be provided between the supports19,120. Llor example, an insulating bushing 21 which may..` be ofceramic or other suitable material may be utilized as a. spacer betweenthe inner ends of supports 19. and 20; Bushing or spacer 21 may beshouldered asl shown -so as to provide additional support for supportring 17. An'annular metallic member 22 which maybe .readilysealedtofglass is joined as by brazing or the like Vto support member20vadjacent the outer end thereof.

Member 22 is sealed as by means of glass bushing 23 to another annularmetallic member 24 which also may .and iron.

Support member 19 extends outwardly beyond member l20 and its outwardlyextending portion or, as shown, a

terminal cap 25 serves as a Contact or terminal for one of the filamentheater connections. As shown, the terminal member or cap 25 may have anouter diameter which is substantially equal to that of the outwardlyextending portion of support 20 which serves as the terminal member forthe other side of the heating current circuit. Intermediate the ends ofsupport 20 and inwardly of the portion thereof which serves as aterminal, an annular member 26 is sealed thereto and serves to supportan annular terminal member 27 which is somewhat U shaped in crosssection. Members 26 and 27 form part of the envelope of tube 10 and areinsulatingly joined together as shown at 28 by glass or other suitablematerial.. It should be noted that terminal vmember 27 forms theinnermost portion of a re-entran part ofthe envelope of tube 10 asshown.

A tubular or cylindrical conductive member 29 is joined to and supportedon terminal member 27 and extends inwardly and engirdles the endportions of support members 19 and 2l) as well as support rings 17 and18 which extend inwardly of terminal member 27. Aiplurality of filaments30, one of which is most clearly shown in Figure 3, are provided. Eachfilament 30 has a pairof parallel portions or strands 31 and 32. The`free end of each of the strands 31 is connected to the end portion ofone of the support rods 16 as by means lof molybdenum clips 33 which arewelded thereto. The free end of each of the strands 32 is similarlyconnected to one of the support rods 16a. In the present instancefilaments 30 are each unitary and are doubled back somewhat in theV formof a hairpin to form the parallel extending portions 31, 32. Each of thefilaments 30 is connected at its midpoint to conductive member 29 bymeans of thin flexible radio frequency connectors or lstraps V34 whichmay be tantalum or other suitable refractory or high melting .pointmetal. The flexible straps orconnectors while serving to rigidify andstrengthen the filament lstructure also permit elongation andcontraction of the filaments without causing deformation. As clearlyshown portions 31 and 32 of each of the filaments 30 extend back alongsupport rods 16, 16a. The array formed by the support rods'and the arrayformed by the filament strands are thus arranged in sideby-'side spacedrelation. As will be more fully pointed out,..such a filament structuremay be made so as to bodies several important advantages.

have such an extremely short electrical length as is desirable andnecessary at the higher frequencies.

An annular terminal member 35 has one peripheral portion insulatinglysealed to a peripheral portion of terminal member 27. Terminal member 35may serve as part of the electrical lead-in and support for controlelectrode 13 which may be supported therefrom as shown byconductivetubular member 36. rlhe other peripheral portion of terminalmember 35 is insulatingly sealed to terminal member 37 which is alsoannular and serves to similarly support screen electrode 14 by means ofconductive tubular member 38.

While the radiator formed by fins 15 may serve as the anode terminal, inthe present instance terminal member 39 is preferably provided. Terminalmember 39 has one end portion thereof sealed to anode 12 while the otherend thereof is sealed to one end of tubular insulator 40 which may beglass. The other end of tubular insulator 40 is sealed to an end portionof terminal member 37.

The envelope and terminal structure of tube 10 em- Terminals 27, 37 and39 are all interposed in the envelope wall and are preferably of highlyconductive material or plated with silver or other suitable highlyconductive material. Such terminals provide low impedance paths andparticularly provide low impedance paths to `high frequency currents.yAll of the terminals including also the filament heater terminals 20and 25 are arranged to facilitate the axial mounting and dismounting ofthe tube in its circuit. A

Another important advantage of such a tube structure resides inthe'relatively short overall axial length which is made Apossible by there-entrant arrangement shown. More particularly the re-entrant structureis provided by the Vfact that all of the filament terminals includingthe high frequency terminal 27, are nested within other parts of theenvelope structure. Tubular insulator 40 which necessarily must berelatively long to afford proper insulation between terminals 39 and 37does not add to the overall axial length of tube 10. This is madepossible because of the arrangement of terminal 35 as'well as most ofthe filament support structure within tubular insulator 40. As shownterminal 35 is of substantially smaller diameter than the diameter ofinsulator 40 and is nested within insulator 40 and adjacent the portionthereof connected to terminal 39. The radio frequency filament terminal27 is seen to be alsoV nested within insulator 40 and is substantiallyin alinement with anode terminal 39. Filament heater terminal 20 is seento be located entirelyl within insulator 40 while `the yother heaterterminal 25 is substantially alined with terminal 37. Thus, it isapparent that the structure combines lthe advantage of long glass orleakage paths with short overall tube or lead length.

A further advantage accrues from the construction of radio frequencyterminal 27. Because of the well known skin effect at the highfrequencies, losses which occur along a conductor increase not only withpower but also Vwith frequency. In the case of radio frequencyconductors which are sealed through insulators it is necessary to avoidexcessive power loss. Such power loss is vmanifested by the generationof heat in the conductor, which, if excessive, results in damage to ordestruction of the seal. In the present instance the radio frequencytermi.- nals or leads which are sealed through orform part of theenvelope wall of tube 16 are readily provided with sufficient surfacearea to avoid excessive losses. In particular, the filament radiofrequency terminal 27 and the seals associated therewith may be madelarge enough in diameter in accordance with the present invention so asto prevent excessive sealvheating and circuit losses. A l Anotherimportant advantage, accrues fromthelfact thatsucha tube may be mountedin its circuitwith conmentary cathode and another electrode, a pluralityof elongated conductive support members in an annular array, saidfilamentary cathode comprising a plurality of filament strands in anannular array, said array of filament strands being interposed betweensaid other electrode and said array of support members, heating currentlead-in and 'support means connected to said support members adjacentone end thereof for applying heating current to said filament strands,each of said filament strands being connected adjacent one end thereofto the other end portion of one of said support members, a leadin andsupport for said other electrode, and a lead-in for radio frequencycurrent interposed in said envelope between said heating currentlead-ins `and the lead-in for said other electrode and connected to theother end portions of each of said filament strands.

2. An electron tube comprising an envelope, a plurality of electrodesspaced in said envelope and including a plurality of cathode filamentstrands in an annular array with the remaining electrodes all on oneside of said array, an annular cathode terminal member interposed insaid envelope, a tubular conductive member connected to said cathodeterminal member `and to each of said filament strands adjacent one endthereof, heating current lead-ins on one side of Isaid cathode terminalmember providing the sole heater current path, a plurality of conductivesupport members connected adjacent one end thereof to said heatingcurrent lead-ins and forming an annular array on the side of saidcathode filament array away `from said remaining electrodes, saidsupport members extending into said envelope beyond said tubularconductive member, and each of said filament strands being connectedadjacent the other end thereof to the inner end portion of one of saidsupport members.

3. An electron tube comprising an envelope, a plurality of coaxial,concentric electrodes spaced in said envelope and including a pluralityof cathode filament strands in an annular array, an annular cathodeterminal member for radio 'frequency current interposed in saidenvelope, a tubular conductor for radio frequency current connected tosaid cathode terminal member extending adjacent to one end of saidfilament strand array and connected thereto, heating current lead-insfor supplying heating current to said filament strands and extendinginto said envelope interiorly of said cathode terminal, a plurality offilament support members in an annular array coaxial and concentric withsaid filament strand array a-nd within the same, said support membersextending through said tubular conductor with the inner ends thereofspaced axially inward of said tubular conductor, and the end portion ofeach of said filament strands remote `from said tubular conductor beingconnected to the innerend portion of one of said support members.

4. An electron tube as defined in claim 3 wherein said filament strandsare substantially hairpin-shaped, and said tubular conductor isconnected to substantially the electrical center of each of saidstrands.

5. An electron tube comprising an envelope, a plurality of cathodefilament strands in an annular array, coaxial concentric control andanode electrodes spaced about said cathode array, annular cathode,control and anode terminal members interposed in said envelope in thatsequence and insulated one from the other, the diameter of said cathodeterminal member being the smallest, the `diameter of said anode terminalmember being the largest, the diameter of said control electrodeterminal member being intermediate that of said cathode and anodeterminal members, concentric coaxial tubular conductive membersconnecting said cathode and control electrode terminal membersrespectively to one end of said filament strand array and to saidcontrol electrode, and heating current supply and support means formingwith said filament strands the sole path for heating current, said meansincluding a pair of heating current lead-ins extending through saidenvelope interiorly of said cathode terminal member, a plurality ofconductive support members in an annular array and connected adjacentone end thereof to said lead-ins, said support members extending throughthe tubular conductive member connected to said filament strand array,the end portion of each of said lament strands remote from 4the tubularconductor connected thereto being connected to the inner end portion ofone of said support members.

6. An electron tube comprising an envelope, a plurality of electrodesspaced in said envelope and including a filamentary cathode and anotherelectrode, a plurality of elongated conductive support members in anannular array, said filamentary cathode comprising a plurality offilament strands in an annular array, said array of filament strandsbeing interposed between the other electrode and said array of supportmembers, a pair of coaxial concentric tubular heating current lead-inssealed through said envelope, support means connected to said heatingcurrent lead-ins and to said support members adjacent one end thereof,each of said filament strands being connected adjacent one end thereofto the other end portion of one of said support members, a lead-in andsupport for said other electrode, and a lead-in for radio frequencycurrent interposed in said envelope between said heating currentlead-ins and the lead-in for said other electrode and connected to theend portions of each of said filament strands remote from said supportmembers.

7. An electron tube comprising an envelope, an annular array of aplurality of cathode filament strands, an annular array of a pluralityof conductive support members spaced radially inward of said filamentstrands with said filament strands extending downward along the same,the inner end portions of each of said filament strands being connectedto the inner end portion of one of said support members, a pair ofnested conductive tubular lead-ins sealed through said envelope ininsulated relation, alternate ones of said support members beingconnected to one of said lead-ins, the remaining support members beingconnected to the other of said lead-ins, an annular cathode terminalmember interposed in said envelope and insulatingly sealed to the outerone of said lead-ins, said cathode terminal member engirdling saidlead-ins and being adjacent to the inner ends thereof, and a tubularconductive member joined adjacent one end to said cathode terminalmember and adjacent the other end to the end portions of said filamentstrands remote from said support members.

8. An electron tube comprising an envelope containing a cathode, saidcathode including a pair of cathode filaments, a tubular cathodeterminal and support member connected to said filaments at one endthereof, and a pair of filament terminal members extending in insulatingrelation through the tubular opening of said `cathode terminal andsupport member and connected respectively to said cathode filaments atthe other end thereof.

9. An electron tube comprising an envelope containing a cathode, saidcathode including a pair of cathode filaments, a grid adjacent to saidcathode, a tubular cathode terminal and support member forming part ofsaid envelope connected to all of said filaments at one end thereof, atubular grid terminal and support member surrounding said cathodeterminal and support member, and a pair of filament terminal membersextending in insulating relation through the tubular opening of saidcathode terminal and support member and connected respectively to saidcathode filaments at the other end thereof.

10. An electron tube comprising an envelope containing a cathode, a gridadjacent to said cathode, a tubular radio frequency cathode terminalconnected to said cathode and forming a part of said envelope, a tubularradio frequency grid terminal of larger diameter than said cathodeterminal connected to said grid and forming a part of said envelope, andmeans for conducting heating current to said cathode including terminalmeans surrounded by said cathode terminal, said cathode terminal formingthe inner-most extension of a re-entrant portion of said envelope.

11. An electron tube comprising an envelope, a plurality of cathodeilament strands, grid and anode electrodes spaced from said cathodeiilament strands, a pair of heating current lead-ins extending into saidenvelope at one end thereof and connected to said iilament strands, anannular cathode terminal member surrounding said pair of heating currentlead-ins, an annular grid electrode terminal member surrounding saidpair of heating current lead-ins and having a diameter larger than saidcathode terminal member, said pair of heating current leadins, saidcathode terminal member and said grid electrode terminal member beinginsulatingly sealed to each other and to said anode electrode, saidcathode terminal member forming the inner-most extension of a re-entrantportion of said envelope.

12. An electron tube as dened in claim 5 wherein said heating currentlead-ins, said cathode terminal member, said control electrode terminalmember, and said e anode terminal member comprise a re-entrant portionof said envelope.

13. An electron tube comprising a iilamentary cathode, co-axial andconcentric control, screen, and anode electrodes, support and heatingcurrent lead-in means comprising a first tubular conductor having aterminal portion at one end thereof, and a second tubular memberco-aXial with and having a larger diameter than said first tubularmember surrounding the other end portion of said first tubular member,an annular cathode terminal member surrounding said other end of saidfirst tubular member and said second tubular member and insulatinglysealed to said second tubular member, an annular control electrodeterminal member insulatingly sealed to said cathode terminal member andextending toward said terminal end of said rst tubular member, anannular screen electrode terminal member insulatingly sealed to saidcontrol electrode terminal member extending toward and surrounding saidterminal end of said first tubular member; whereby said iirst tubularmember, said second tubular member, said control electrode terminalmember, and said screen electrode terminal member form a doublereentrant portion of the envelope of said electron tube.

ieterenees Cited in the le of this patent UNITED STATES PATENTS2,399,004 Crawford Apr. 23, 1946 2,451,987 Sloan Oct. 19, 1948 2,452,786ONeil NOV. 2, 1948 2,489,872 Elder et al Nov. 29, 1949 2,510,171 Clarklune 6, 1950 2,534,548 Fay et al. Dec. 19, 1950 2,542,639 De Walt Feb.20, 1951 2,617,960 Kohl Nov. 11, 1952 2,705,294 Schrader Mar. 29, 1955 2726,349 Wing et al Dec. 6, 1955 UNITED STATES PATENT OFFICE CERTIFICATEOF vCORRECTIGN Patent No., 2,841,736 July l, 1958 Merrald Bo Shrader Itis hereby certified that error appears in the-printed specification ofthe above 4'numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column l, line 33, after "a structureu insert the u; column 5,\ line 66,for "each" read each ne; column 8, lines 59 and 6D, strike out nformingpart of said envelope; same column 8, line 60 strike out "all Of oSigned and sealed this 28th day of October 1958.,

(SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner ofPatents

